a network consists of the following list. times are given in weeks. activity preceding optimistic (a) probable (m) pessimistic (b) a 7 9 10 b a 2 2 8 c a 8 12 16 d a 3 7 10 e b 4 6 8 f b 6 8 10 g c, f 2 3 4 h d 2 2 8 i h 6 8 16 j g, i 4 6 14 k e, i 2 2 5 what is the standard deviation for activity e?